Porphobilinogen synthase catalyzes the synthesis of the pyrrole, porphobilinogen, from two molecules of delta-aminolevulinic acid. Four molecules of the pyrrole are condensed to form the heme structure and the corrin ring of vitamin B12. We are continuing our investigation on the structure, function and mechanism of porphobilinogen synthase, an octameric enzyme. This year we are planning to continue our determination of the primary structure of the monomers (m.w. 35,000), the amino acids involved at the active site and the role of Zn 2 ion in the enzyme. Furthermore, we shall continue our subunit-subunit hybridization studies using our previously described immobilized enzyme preparation. This preparation allows us to study interspecies as well as intraspecies hybridization and thus study structure-function relationships. The hybridization of subunits of different species allows us to prepare pure enzymes from a variety of species by a very simple operation. We plan to explore this area further.